1. Field of the Invention
This invention relates to stress relieved end-burning propellant grain structures and, more particularly, to an improvement thereof that makes practicable a very high propellant mass fraction which is also very highly relieved structurally. "Mass fraction" is the ratio of the mass of the propellant grain in a rocket motor case to the total mass that could be put in the case if it were completely filled.
2. Description of the Prior Art
Solid propellant rocket motors of the end or "cigarette" burning type wherein only the exposed end face of the propellant grain burns are known in the prior art. They are commonly used in the aerospace field of activity for providing rocket thrust in a "sustain" mode of operation. Generally, such motors utilize a thin-walled, high-strength cylindrical rocket motor case into which the propellant grain is cast and cured, the case being closed at the forward end and having a nozzle at the aft end for the emission of exhaust gases.
Problems have been experienced with solid propellant rocket motors of the end burning type stemming from a compelling need that, when cast and cured, the propellant grain must be substantially relieved from stress and strain. Such loads tend to be introduced particularly under the temperature changes that occur during solid propellant rocket motor operational requirements. The propellant grain must be free from even minute cracks or fissures which might tend to propagate under operational loading conditions of the motor and cause unscheduled and uncontrollable burning surfaces that can result in an explosion of the motor.
In order to overcome these problems it is the current practice in the prior art to form the end-burning propellant grain from three parallel propellant components that are separated from each other in the motor case by a longitudinal inhibitor having the shape of a "Y" in cross section with a longitudinal hole or port in the center and a longitudinal hole at each of the three tips for providing internal strain relief. After assembly of the inhibitor in the motor case, in which a suitable liner is provided, the propellant components are cast and cured, being separated from each other by the inhibitor. The inhibitor cross sectional area is substantially equal to the thickness thereof times the motor circumference.
While satisfactory for reducing the strain to which the resulting end-burning grain is subjected, this prior art practice is disadvantageous in that the volume and, hence, mass of propellant grain that can be loaded into the motor case is reduced by the space occupied by the holes at the inhibitor center port and tips. Typically, such reduction expressed in percentages is of the order of two percent. This is not an insignificant factor as it can effect a significant reduction in the desired range of the rocket motor.
There is thus a need and a demand for improvement in the structural arrangement of end-burning rocket propellants to enable the attainment of a high degree of relief from strain and concomitantly the achievement of the maximum volume and, hence, mass of propellant that can be loaded into an operational tactical rocket motor. The present invention was devised to fill the technological gap that has existed in this respect.